In a metal gasket formed by laminating a plurality of metal plates, a connecting structure for connecting all or a part of the plurality of the laminated metal plates with each other includes a conventional structure, for example, disclosed in Japanese Patent Publication JP10-235440A. In this structure, laminated metal plates are connected with each other in such a manner that in a connection portion, the laminated metal plates are cut along two main section lines independent from each other to be cut with two lines, and a metal portion between the two main section lines is extended and evaginated to project in the lamination direction (thickness direction), forming a protrusion portion so that the metal portion may provide a larger stepped difference than a thickness of the laminated plates. In addition, in cutting, the plates are punched out with a wider clearance to a mold than usual, and many burrs are generated on the cutting surface, and/or a property of material with high hardness is used to widen the burrs larger than the cutting surface after cutting, and further punched out portions engage with one another due to the widened burrs and may not return to a punched hole, thus the metal plates may be kept locked.
Here, in a conventional art, a metal gasket is provide with a projecting portion in the outer circumference thereof so that a part of the metal gasket is situated outside a connection surface of an engine having the metal gasket mounted, and a connection location is set in this projecting portion to connect. However, as an engine grows smaller in size and more sophisticated, and according to adoption of a wet liner or open deck technology in a cylinder block as a cooling method against a high temperature, and further in order to take a countermeasure against lowering in bearing stress generated around a combustion chamber hole, a gasket which raises the bearing stress by installing more shim plates around the combustion chamber hole has been increasingly used. Considering these circumstances, the connection method above has been used when a plurality of metal plates constituting a gasket are connected with each other in a region where a water conduit opening is provided around the combustion chamber hole.
However, if the connection location is set in a region where a water conduit opening is provided in the connection surface of an engine, a crack may occur from the relevant connection location depending on conditions of an intended engine, because cyclic, tensile and compressive load is applied due to engine operation.
For example, a compression ratio and/or combustion temperature etc. is raised in an engine which is miniaturized, sophisticated and/or has a large-volume, etc., it is required to increase a cooling efficiency of a cylinder block and decrease a manufacturing cost, and a wet liner or open deck technology etc. is adopted, so as the result, in a cylinder block of an engine, a combustion chamber hole's side of the engine and a bolt-hole portion outside a water conduit opening are separated at a location for forming a water conduit opening in a connection surface opposed to a metal gasket, where the combustion chamber hole's side and an outer portion from the location for forming a water conduit opening are communicated with each other through a bottom of the water conduit opening. Further, a metal gasket mounted between the cylinder block and a cylinder head of such an engine is arranged to receive the highest load (bearing stress) applied to the gasket around the combustion chamber hole. In addition, the outside of the location for forming a water conduit opening is fastened with a clamping bolt, then load applied near a bolt-hole on the outside straddling the location for forming a water conduit opening becomes approximately equal to that around the combustion chamber hole, and the bearing stress decrease gradually apart from the bolt-hole, though in a different manner depending on rigidity of the engine except in a region near the bolt-hole. Under these conditions above imposed concurrently, it has been observed that because the combustion chamber hole's side of the location for forming a water conduit opening and the outside of the location for forming a water conduit opening are separated at the location for forming a water conduit opening, they move differently from one another depending on conditions in engine operation.
At this time, in the conventional connecting structure, work is applied in which, so as to leave behind outsides of a base plate within a range to be connected, two or more notches are cut out between the outsides to generate a burr, and the plate on the side cut out is extended to be evaginated in shape of trapezoid having a partially flat portion in the central portion. Particularly, when a gasket is formed of material with high rigidity, because a connection location is set inside a location for forming a water conduit opening (a location overlapped with a water jacket of a cylinder block), a size of the connection location is limited, and when a connection portion is small and the evaginated portion is evaginated beyond a thickness of the gasket, then its rising-edge portion is extended forcibly to have residual tensile stress.
Further, in a method of the relevant conventional technology, because a shear plane finally cut out is perpendicular to a rising-plane of the gasket plane, tensile stress is generated to concentrate in a cutting surface formed by final shearing. When the gasket under these conditions is mounted on an engine and fastened with a clamping bolt, and the engine is operated, then a portion connected, which is released in both of a cylinder block surface and a cylinder head surface, or only in the cylinder block surface within a range of a water conduit opening, in either case, is in a freely movable state within the range of a water conduit opening, and when the engine moves differently between in an inner circumference and outer circumference of the water conduit opening as described above, then in a connected gasket within the location for forming a water conduit opening, a crack may occur in a gasket base plate from a base point which is the cutting portion by final shearing where stress stays behind, due to compressive and/or tensile stress and vibration repeatedly produced from operation. Development of the crack with time may damage a function of the gasket.